Facies Architectural Analysis of the Shallow Subsurface: A Strategic Component of an Interdisciplinary Approach to Hydrogeologic Site Characterization

Kathleen M. Farrell, Richard M. Wooten, Hal C. Bryson

A qualitative assessment of groundwater flowpaths in the shallow subsurface (<60 feet) requires an understanding of the facies architecture of the sedimentary sequence underlying a site and its relationship to the three-dimensional distribution of key hydrologic parameters (porosity, permeability, and hydraulic conductivity), and diagenetic, weathering and structural features. An interdisciplinary approach to hydrogeologic characterization includes: 1) integrating geomorphic, geologic, geochemical, and geotechnical investigations to develop and refine information on facies architecture, and 2) using the architectural analysis as a predictive tool to locate and design hydrologic tests.

In an unconsolidated, surficial, inner Atlantic Coastal Plain sequence (North Carolina) interpreted as a series of regressive barriers (Pinehurst Fm.) that overlie Cretaceous fluvial deposits (Middendorf Fm.), groundwater and interflow pathways in the shallow subsurface are probably controlled by geomorphology, the facies architecture of inlet, channel and barrier sands (primary permeability), and associated clay-bodies (that cause perching), the Pinehurst-Cretaceous contact, and possibly secondary fracture porosity associated with structural features. In this setting, characterization of the shallow subsurface is fairly straight forward.

The shallow subsurface is particularly difficult to characterize hydrogeologically, however, in a structurally-deformed, Triassic-age, alluvial sequence in the humid southeast, where surface weathering obliterates outcrops and produces relatively thick weathered zones that are not easy to sample and do not resolve well in geophysical investigations. In this setting, water can travel laterally as interflow in the vadose zone above low-permeability horizons and through secondary porosity features such as fractures and bedding planes that are enhanced by surface weathering and penetrated by macropores.

New areas of research in diagenesis should focus on characterizing diagenetic patterns and fabrics in weathered, siliciclastic rocks and understanding the relationship between these patterns, facies architecture, and hydrologic processes within the saturated and unsaturated zones. Locating and describing the permeability architecture, especially secondary fracture porosity and secondary dissolution porosity, in weathered rocks and sediments of the shallow subsurface is difficult, but is necessary to assess flowpaths for potential leachates from disposal facilities and to develop monitoring networks to detect such releases.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995